Gap Mountain, just outside of Fitzwilliam, NH, is the perfect mountain for hiking with a baby on your back.  With a decent walking stick and good shoes (and, of course, a sturdy and comfortable backpack carrier), it's just steep enough to feel like more than a walk in the woods, but still easy enough that you won't mind your center of balance being thrown off by 22 lbs of toddler on your back.  It takes about 90 minutes to get from the parking lot up to the top of the mountain and back.

It had rained a few days before we got there (the day after Thanksgiving), so unfortunately there were too many icy rocks near the top for me to make it all the way up.  Fortunately, D had fallen asleep in the Ergo and didn't mind waiting about 20 minutes for the rest of our party to go up to the summit and back.  I found a nice flat rock to sit on, rocking back and forth so D could finish his nap, where I was passed by 3 or 4 families with older kids (age range from about 7 to a couple of teenagers) on their way to the peak. 

In the late summer, there are blueberries to pick at the top, but in case global warming or bears have gotten to them before you do, it's a good idea to bring some snacks along with your water.